Imagewise sensitization of electro-photographic layers



United States Patent.

2 Claims. (c1. 96-1) The invention relates to a process for sensitising electrophotographic layers, mainly layers of zinc oxide and a binding agent, by means of which mosaic sensitization or image sensitization is possible.

In the known and hitherto most customary sensitizing process for ZnO layers with resin binding agents, an alcoholic solution of the sensitizing dyestufi' is stirred or ground with photoconductive ZnO, the solvent is then removed by drying, and the slightly colored zinc oxide is mixed with the insulating binding agent in the usual manner and the dispersion cast onto the support. There are numerous variations of this method, for example, in one case the sensitizing dyestuli is not previously dissolved in alcohol but added to the solvent for the binding agent, e.g., toluene, and ground in this form with zinc oxide. In another method, the unsensitized electrophotographic paper is bathed in a solution of sensitizing dyestuff. Another process for preparing sensitized layers comprises applying two superimposed ZnO.layers onto the carrier, the outer of the two layers containing the sensitizing dyestuff. This method is used for obtaining maximum sensitization with low coloration.

With the sensitizating process, uniformly sensitized electrophotographic layers are obtained which have a weak coloration and whose dark resistance is in most cases lower than in uncolored ZnO layers.

Furthermore, it is known to apply the sensitizing dyestuff not uniformly but differentiated in a screen pattern. This is accomplished by means of printing processes known per se. However, the utility of this method is limited since only Wide screen patterns can be printed, moreover in the most cases the electrophotographic layer is softened, whereby the electrophotographic as Well as the mechanical properties of the layers are impaired.

- It is among the objects of the instant invention to avoid the disadvantages described above and to provide a process for preparing electrophotographic layer which are imagewisely sensitized.

We now have found that these objects can be attained by applying the sensitizing dyestufl by means of electrophotographic developing process to the unsensitized electrophotographic layer carrying a latent electric image.

The development is accomplished with a solution or dispersion of the sensitizing dyestulf which is not colored or is only very weakly colored. A practically invisible image of the sensitizing dyestulf is thereby formed on the electrophotographic layer. The application of the sensitizing dyestufi on the exposed material may be carried out by known developing processes. For example, the dry powder of a sensitizing dyestulf may be used in the usual powder process, employing the development of cascades or dust clouds, and the powder, which is deposited in a pattern corresponding to the image, may be fixed by after-treatment with a solvent. Suitable processes are described, for example, in US Patents 2,297,691; 2,618,552; 2,725,304 and British Patents 793,331 and 835,044.

Other known electrophotographic developing processes 7 may also be used, for example, the wetting processes described in Belgian Patent 616,644 in which the different wetting capacity of electrophotographic layers which have been charged in patterns corresponding to the image or which have been rendered conductive by exposure in patterns corresponding to the image is utilized for development. In the present case, an uncolored or only slightly colored solution of sensitizing dyestuff is used instead of the deeply colored dyestuff solution.

Particularly suitable for the application of the sensitizing dyestuif are the developing processes in which a colored aerosol is used, preferably one that has been produced by electrostatic means. The electrostatic charging of the aerosol thereby effected provides a particularly advantageous deposition of the sensitizing dyestutf on the exposed electrophotographic layer. Developing processes of the last mentioned type are described, for example, in Belgian Patent 616,914. As already described there, the highly concentrated dyestulf solutions which may be used for'development of the picture are more easily atomized with positive atomizing electrodes than with negative electrodes, and much simpler electrode arrangements may be used for positive atomization. The droplets of liquid formed are positively charged. In the known process for electrostatic image development by the processes of negative charging-exposure-development, a positive image of the original isalways obtained when using a positively charged aerosol. A reversed image, that is to say a negative, could be produced under these conditions only if a positive charged image could be produced on the layer, The positive charging instead of negative charging in the first stage of development cannot be carried out to the same degree of charge and quality as the negative charging, owing to the specific electric properties of zinc oxide, so that it is difficult to reverse the original picture by charging the layer positively. However, in many cases, it would be very important to obtain a reversal from a negative to a positive image, for example, in the production of enlargements of negative films.. In these cases, a positive developing process by means of aerosol sensitization according to the invention may be employed. According to the process described in Belgian Patent 616,944, electrostatic charged images may be developed by means of colored aerosols with the aid of an electro-- static atomizer electrode combined with a grid shaped developer electrode. Due to the low concentrations of a sensitizing dyestuff instead of the highly concentrated dyestuif solutions, the resulting image is invisible. However, the quantity of sensitizing dyestuff applied is sufficient to increase the photoconductivity of the zinc oxide layer or to render the photoconductive layer, sensitive to other regions of the spectrum at the developed and sensitized areas.

The process described above provides an electrophotographic material whose photoconductive substance has been sensitized by the differential application of sensitizing dyestuff distributed in accordance with the image. The final picture is now produced by charging the layer again and then exposing it uniformly, i.e. not imagewise. This produces a charged image which is negative to that of the sensitizing dyestuif, and this charged image is rendered visible by means of a blackening or deep coloring developer recording to common practice. Any of the usual processes may be used for this purpose, but development with a colored aerosol is again preferred.

When using a colored aerosol process for the final development, the electrophotographic material, sensitized in a pattern corresponding to the image by the process accord-ing to the invention would be charged negatively, exposed uniformly and developed in the usual manner, for example by the colored aerosol processes described in the above mentioned Belgian patent. A positive picture of the negative original is thereby obtained.

The invisible intermediate sensitization image, which is characterised by the feature that the sensitivity of the zinc oxide layer is altered in accordance with the image, is stable a very long time and may, for example, be left for several months before it is finely developed even if the layer has in the meantime not been kept away from the light, provided that the light is not so intense that it decomposes the sensitizing dyestutf. The only condition that must be observed to ensure that the image may be rendered visible is that, for the reasons already known, the layer must be stored in darkness for some time before the picture is developed.

In addition to the image reversal from negative to positive image, a substantial improvement of the electrophotographic layers as regards the half-tone development in general and as regards the uniformity of blackening (avoidance of marginal effects) is provided since a screenlike sensitization can be applied by the use of the method according to the invention. Screen sensitization similar to that obtained in the above described image sensitization of electrophotographic zinc oxide layers may easily be obtained by exposing a screen and subsequently developing with a sensitizing dyestuff. The concentration of the dyestuif in the solution may be kept so low that the production of the screen does not lead to any discoloration of the surface of the layer.

Screenlike sensitized electrophotographic layer or papers have great advantages compared with non-screened layers. For example, the range of exposure is considerably increased, and a screened charged image may be produced by a simple exposure of the previously charged layer. Owing to the inhomogeneity of the electric field distributed over such a layer, uniform blackening is obtained over large surfaces even with toner dust development. Furthermore, a multiple screen may be produced with sensitizers for different wavelengths of light. In this case, the differently sensitized surface elements of the layer will not or will only partly coincide with one another. This type of screen formation is of greater importance for the development of multicolor images because the colors will not be deposited over one another but on minute surface areas side by side.

Preferred are aerosol development processes since only these processes are suitable for the production of very fine screens, which are important for practical purposes. The screenlike sensitization which can be obtained by the inventive process may correspond to a screen having of between 100-200 lines/cm.

Another advantage of the sensitization according to the invention is that, starting from a uniformly produced white photoconductive layer with high dark resistance, the sensitization may 'be controlled as desired by varying the concentration of the deposition of sensitizer or by mixing different sensitizers. These measures may be effected particularly simply by spraying the sensitizer with the aid of one of the known colored aerosol processes, preferably electrostatic aerosol processes.

Example 1 320 g. of a silicone resin (e.g., type Bayer P150) as a 60% solution in toluene, 5100 g. of toluene and 450 g. of chemically pure photoconductive zinc oxide are ground for about 3 hours in a ball mill and then cast on to a paper and dried. The electrophotographic paper so produced is given a negative electric charge in known manner by means of a charging apparatus and then exposed through a film by means of an incandescent lamp so that a charged image is produced on the layer. With the aid of the apparatus described in Belgian Patent 616,944 which contains a grid electrode and an electrostatic atomiser electrode, the layer is sensitized in a pattern corresponding to the charged image with a solution of br-omophenol blue in benzyl alcohol in the proportion 1:1200. After storing the paper for 3 hours in the dark, the layer is charged again and exposed diffusely with yellow light of wavelegnth 0.60 to 0.65,. Subsequently development with a solution of Ceresblack in benzyl alcohol in the proportion 4:7 using the apparatus described in the last mentioned Belgian patent produces a reversed black image of the original.

Example 2- Screen sensitizati0n.An electrophotographic paper according to Example 1 is given a negative electric charge by means of a charging apparatus and exposed to light through a film which has a cross line screen (5 lines per cm.), which is placed on the layer. The electric screen image thereby formed is sensitized by the aerosol of a solution of fluorescein in benzyl alcohol in the proportion 1:2000, using the same apparatus as in Example 1, The electrophotographic layer is sensitized in a screen pattern. Similar results are obtained if line or cross line screens are used for the exposure which have -200 lines/cm.

After storing the paper for 3 to 5 hours in the dark, it may 'be processed in the usual manner. That is to say, it is charged and then exposed through the original. The light used for the exposure is one to which both the sensitized and the non-sensitized region of the electrophotographic layer are sensitive although to different extents. The picture may be developed either by aerosol process or by the toner process.

The process according to the invention can be varied in many respect. For example, electrophotographic layers other than those given in the examples may be used. The silicone resins may be replaced by other bin-ding agents such as alkyd resins, formaldehyde or phenol resins or other binding agents. Instead of zinc oxide, other organic or inorganic photoconductive substances may be used.

Furthermore, the process according to the invention is independent of the type of sensitizing dyestuff used. Any of the known sensitizers may be used, for example:

Dyestuff Colour Index Fluorescein 45350 Eosine 45380 Bengal pink 45435 Malachite green 42000 Crystal violet 42555 Methyl green 42590 Cryptocyanine.

Pinacyanol.

Methylene blue 5 2015 Acridine orange 46005 Methylene green 50431 Alizarine red S 58005 Quinizarin 58050 Phenolsulphonphthalein (according to German patent specification No.

1,090,093) Bromphenol blue. Chlorphenol red. Bromcresol green. Bromcresol purple. Thioflavine 49005 Brilliant green 42040 Particularly suit-.

layer carrying the charge image is sensitized in a pattern (d) charging again the thus developed photoconductive corresponding to the image, and the layer is then charged layer in the dark, again in the dark, exposed uniformly with light of a wave- (e) uniformly exposing the charged layer with light length to which the sensitized regions of the electrophotoof a wavelength to which the sensitized regions of graphic layer are sensitive, and finally developed in the 5 the layer are sensitive, and usual manner, whereby a picture is obtained whose grada- (f) finally developing the exposed layer in the usual tion is the reverse of that of the original. manner, whereby a picture is obtained whose grada- 2. A process for the production of electrophotographic tion is the reverse of that of the original. images comprising the steps of (a) negatively charging a supported photoconductive 10 References Cited y the Examine! y UNITED STATES PATENTS (b) imagewise exposing the charged photoconductive I layer, to form a latent charge pattern in accordance ag ii; with the image to be reproduced, 3O03870 10/1961 J (0) developing the photoconductive layer bearing the 15 arvls at a 9'6 1 charge pattern with an electrical charged aerosol 3005726 10/1961 1 252 62 1 X of a solution of sensitizer whereby the aerosol is de- 3,041,169 6/1962 wleilckl 96-4 posited in accordance with the latent charge pattern, 3960920 10/1962 Grelg and whereby the photoconductive layer is sensitized in a pattern corresponding to the image exposed 20 NORMAN TORCHIN Pr'mary Examiner there-on, C. E. VAN HORN, Assistant Examiner. 

1. A PROCESS FOR THE PRODUCTION OF ELECTROPHOTOGRAPHIC IMAGES, WHEREIN AN ELECTROPHOTOGRAPHIC MATERIAL IS GIVEN A NEGATIVE ELECTRIC CHARGE, EXPOSED THROUGH AN IMAGE, DEVELOPED WITH AN ELECTRICALLY CHARGED AEROSOL OF A SOLUTION OF A SENSITIZER, WHEREBY THE ELECTROPHOTOGRAPHIC LAYER CARRYING THE CHARGE IMAGE IS SENSITIZED IN A PATTERN CORRESPONDING TO THE IMAGE, AND THE LAYER IS THEN CHARGED AGAIN IN THE DARK, EXPOSED UNIFORMLY WITH LIGHT OF A WAVELENGTH TO WHICH THE SENSITIZED REGIONS OF THE ELECTROPHOTOGRAPHIC LAYER ARE SENSITIVE, AND FINALLY DEVELOPED IN THE USUAL MANNER, WHEREBY A PICTURE IS OBTAINED WHOSE GRADATION IS THE REVERSE OF THAT OF THE ORIGINAL. 